In order to ensure error-free data transmission, the pulse form at the output of a transmitter (Line Interface Unit) must comply with specific specifications (see CCITT G.703 Physical/Electrical Characteristics of Hierarchical Digital Interfaces). Appropriate standard pulse masks, which the transmitters have to comply with, are specified as a function of the data rate and of the chosen Standard.
Transmitters which have been developed to date have been able to satisfy the specified Standards satisfactorily only when sufficient operating voltage was available. The concept of such known line interface units is predominantly based on voltage-processing principles in the signal path of the transmitters, which may have resulted in certain fluctuation in the operating points in the corresponding circuits, and entailing restricted-quality operation of the overall transmitter. Furthermore, these principles are only inadequately suitable for modern deep submicron CMOS processes and the withstand voltage, which is restricted as a consequence of this, of regular MOS transistors.
Furthermore, the circuits of known transmitters have relatively high-value mode impedances, which in turn result in severe thermal noise and thus have a limited signal-to-noise ratio. Apart from this, the known transmitters were sensitive to offset voltages, which cause an undesirable direct current via the load at the output. In particular, the prior saturation of a downstream transformer can lead to additional undesirable distortion of the signal.
A range of publications exists in which voltage-processing transmitters are described. These include, for example:    H. Herrman and R. Koch: “A 1.544 Mb/s CMOS Line Driver for 22.8Ω Load”, IEEE Journal of Solid-State Circuits, June 1990, Pages 760 etc.,    Haideh Khorramabadi: “Highly Efficient CMOS Line Drive with 80 dB Linearity for ISDN U-Interface Applications”, IEEE Journal of Solid-State Circuits, December 1992, Pages 1723 etc., and    M. Moyal, M. Gröbel and Th. Blon: “A 25 kft, 768 kb/s CMOS Analog Front End for Multible-Bit-Rate DSL Transceiver”, IEEE Journal of Solid-State Circuits, December 1999, Pages 1961 etc., and the references cited in these documents. The known solutions, however, are subject to all of the disadvantages that have been mentioned above.
The object of the present invention is thus to provide a transmitter for transmission of digital data, which allows the generation of pulses of considerably higher quality and accuracy, and which avoids the disadvantages of voltage-processing principles for signal generation.